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Abstract

A new species of the genus Pichia has been recovered 38 times in the Sonoran Desert from “rot pockets” of cereoid cacti and from Drosophila species which utilize the cacti. We have named the species Pichia amethionina due to its absolute requirement for methionine or cysteine. P. amethionina is heterothallic and demonstrates an agglutination reaction when opposite mating types are mixed. Two varieties are designated based on the combination of mannitol assimilation and ecological habitat. P. amethionina var. amethionina, the type variety, was recovered from cacti in the subtribe Stenocereinae and cannot assimilate mannitol, whereas P. amethionina var. pachycereana was found in cacti of the subtribe Pachycereinae and can assimilate mannitol. Results are given which demonstrate that the assimilation of mannitol is controlled by a single genetic locus. An evaluation of the interfertility and postmating viability among the two varieties and possibly identical organisms was made. The base composition of the nuclear deoxyribonucleic acid (average of 10 strains) is 33.05 ± 0.19 mol% guanine plus cytosine. The type strain of P. amethionina and of the type variety, P. amethionina var. amethionina, is UCD-FST 76-401B (=ATCC 36080 = CBS 6940). The type strain of P. amethionina var. pachycereana is UCD-FST 76-384A (=ATCC 36079 = CBS 6943).

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